Pyromellitic acid dianhydride (abbreviated hereinafter to PMDA) is an important starting product in the industry for the manufacture of special plastics. Polyimides or polyimide azopyrrolones prepared by the polycondensation of PMDA with aromatic diamines or tetramines have long been known as superior heat-resistant electrical insulating materials which are used in the form of castings, moldings, films and sheets and as coatings for electrical wires and cables. PMDA of high purity is required as the starting product for the preparation of these polyimides or polyimide azopyrrolones.
PMDA is prepared, for example, by catalytic gas phase oxidation of Durol (1,2,4,5-tetramethylbenzene) or other 1,2,4,5-tetraalkylbenzenes with gas containing oxygen, especially air, in the presence of catalysts. The impurities which PMDA made by this method generally contains are, for example, mono-, di- and tribasic acids or anhydrides, colored substances, etc.
In the state of the art there are a variety of processes for the refinement of crude PMDA prepared by gas phase oxidation of 1,2,4,5-tetramethylbenzene or other tetraalkylbenzenes. For instance, Belgian Pat. No. 676,048 proposes recrystallization from aromatic hydrocarbons with intermediate complexing, Netherlands Offenlegungsschrift No. 6,509,701 proposes recrystallization from dioxane or methyl isobutyl ketone, and U.S. Pat. No. 2,578,326 proposes vacuum sublimation from a suspension of the anhydride in inert, heat-resistant organic liquids. From Netherlands Offenlegungsschrift No. 6,516,840 it is furthermore in the prior art to perform the refinement by bubbling hot reaction gases through at elevated temperature. Also known is a process in which the vapor-gas mixture leaving the oxidation furnace is passed through water washers. In this manner the anhydrides are hydrated to the corresponding acid and the acid is dissolved. However, the impurities which develop during the oxidation also pass over into this solution. This is again followed by special refining processes requiring apparatus protected against corrosion. After the refinement the acid has to be dehydrated again, additional expensive operations being required for this purpose.
The known processes require large investments in apparatus and require large amounts of energy and time; furthermore, in some cases they deliver a very poor yield of pure product.